Radiotelephones, which are well known in the art, generally refer to communications terminals which can provide a wireless communications link to one or more other communications terminals. Such radiotelephones are used in a variety of different applications, such as cellular telephone systems and commercial and military satellite communications applications. As radiotelephone communications technology has matured and the uses for such terminals proliferated, significant advancements have been achieved in reducing the size, weight and cost of radiotelephones while improving their performance. However, while these advancements have provided a variety of relatively compact radiotelephones, significant demand still exists for both smaller and lighter radiotelephones and for radiotelephones which provide enhanced signal quality or additional capabilities. Tempering these demands are cost considerations, as the availability of other forms of communication limit the amount many potential consumers are willing to pay for the additional convenience of smaller handsets or improved communications quality.
The size and weight of a radiotelephone are typically increasing functions of the radio frequency transmit power which the radiotelephone is designed to provide. This required transmit power is determined by the link budgets associated with the communications systems in which the radiotelephone is designed to operate. These link budgets typically provide link margins to account for a variety of fixed or varying losses, which may include propagation losses and the losses associated with both the transmit and receive antenna systems. Accordingly, to the extent that these propagation and antenna system losses can be minimized, the required transmit power typically is reduced thereby reducing the size and weight of a given radiotelephone.
One such propagation loss occurs when a mismatch exists between the polarization of the received signal and the polarization of the receive antenna. As most radiotelephones employ linearly polarized (i.e., horizontal or vertical) dipole antennas, the opportunity for mismatch between the polarization of the signal received at the antenna and the polarization of the receive antenna is quite high, as the antenna on the handheld radiotelephone is typically not stationary, and hence may often be held at an orientation different than the orientation of the antenna on the radio which transmitted the signal. Such polarization mismatch may also occur (or be further aggravated) as a result of obstructions on the line-of-sight path between the transmitting and receive terminals, which can alter the polarization orientation of the signal during propagation. While the extent of the loss in receive signal power which occurs as a result of polarization mismatch varies significantly depending on orientation, in the worst case it can be quite severe.
Another potential propagation loss which may be accounted for in the system link budgets (and transmit power requirements) is the loss to signal power which may occur during propagation due to atmospheric conditions, such as rain or snow, or due to other physical obstacles which reflect radio frequency energy. These "reflection" losses are path dependent (i.e., the losses are a function of the actual path the signal traverses during transmission), as they are based on the conditions encountered by the waveform in propagating from the transmit antenna to the receive antenna.
The extent to which a receive signal is degraded as a result of polarization or reflection losses tends to vary depending upon a number of factors, including the geography, the type of antenna on the terminal communicating with the radiotelephone (e.g., fixed orientation or variable orientation) and the atmospheric conditions. Accordingly, depending upon a users location and the type of radiotelephones used, a user may or may not be concerned with polarization and/or reflection losses.
A third type of loss which impacts the transmit power a radiotelephone is required to provide is the signal loss incurred in the antenna feed structure on both the transmit and receive terminals. On most conventional radiotelephones, this feed structure includes an antenna duplexer which functions to channel energy received from the antenna to the receiver during periods of reception and to couple signals from the transmitter to the antenna during periods of transmission. Such duplexers typically incur a loss in signal power on the order of 2-3 dB.
Each of the above-mentioned losses in signal power must be accounted for in establishing the link budgets for the communications system in which the radiotelephone is to operate. This is done by designing the system so that sufficient signal power will be received even when one or more of these losses occur, thereby ensuring that an acceptable communications link may be established in the majority of situations. To provide this additional power margin in the link budgets, system designers typically require increased transmit power on each transmitting terminal or attempt to provide increased transmit or receive antenna gain through the use of a directional antenna. However, providing such increased transmit power or upgraded antennas may negatively impact the size, weight and cost of the radiotelephones used in the communications system.
Various researchers have recognized that by providing a second antenna on a radiotelephone, it may be possible to reduce certain losses in receive signal power. However, these researchers have failed to identify a system for managing such a second antenna in a way that permits tapping its full capabilities without the unproductive duplication of circuitry.
In addition to consumer demand for smaller and lighter radiotelephones, there is also a significant demand for radiotelephones with additional capabilities. In response to this demand, the industry is in the process of transitioning from an environment where almost all radiotelephone communications were provided via mature and relatively inexpensive analog technology to digital service which is capable of providing additional features and improved performance. These digital systems make more efficient use of the bandwidth available for various types of radiotelephone communications, provide additional features such as communications security and often take advantage of error correction coding, interleaving and other techniques to provide enhanced signal clarity as compared to their analog counterparts. However, while use of digital radiotelephones is becoming increasingly prevalent, the transition from analog to digital service may take years or even decades to occur. Consequently, mobile radiotelephone users, such as cellular telephone users, may find that they require a digital radiotelephone in some geographic locations and an analog radiotelephone in others. This situation has resulted in a great demand in the marketplace for dual-mode (analog and digital) radiotelephones, which allow users to purchase a single radiotelephone that is capable of operating in both analog service and digital service regions. However, such dual-mode radiotelephones are typically larger, heavier and more expensive than comparable single-mode radiotelephones due to duplication of circuitry.
In light of the above-mentioned problems with current radiotelephones, a need exists for radiotelephones that are designed to minimize the effects of propagation, reflection and antenna feed losses, thereby allowing the use of smaller and lighter transmit power and power storage (battery) systems in the radiotelephone. A need also exists for dual-mode (analog/digital) radiotelephones which can be configured to provide a high degree of performance in a variety of different communications environments without requiring duplicative circuitry which might increase the size, weight or cost of the radiotelephone.